Isomerization kinetics of carbon clusters 2. Transformation of tricyclic structures to fullerenes

A quantitative model of isomerization of carbon clusters is proposed, and possible ways of transformation of tricyclic structures to fullerenes are considered in order to account for the spontaneous transformation of the polycyclic clusters arising during the condensation of the carbon vapor. The isomerization process is calculated thermodynamically using the modified classical interatomic potential, the relative effeciencies of different mechanisms of the fullerene formation are estimated, and the main regularities of this process are established. The results of the simulation are consistent with the experimental data. The model can be employed in the calculation of long isomerization chains of complex molecules. The accuracy of the quantitative estimation of the kinetics is determined by the errors in the calculations of the bond energies of the isomers. Institute of Theoretical and Applied Mechanics, Siberian Branch, Russian Academy of Sciences. Institute of Computation Technologies, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 37, No. 4, pp. 671–681, July–August, 1996. Translated by I. Izvekova     

Random replicators with high-order interactions

We use tools of the equilibrium statistical mechanics of disordered systems to study analytically the statistical properties of an ecosystem composed of N species interacting via random mutual interactions, as well as via deterministic self-interactions of order p>/=2. We show that the main effect of increasing the order of the interactions among the species is to make the system less competitive, in the sense that the fraction of extinct species is greatly reduced. In addition, we find that for p>2 there is a threshold value which gives a lower bound to the concentration of the surviving species, preventing then the existence of rare species and, consequently, increasing the robustness of the ecosystem to external perturbations.     

Structure of a microparticle crystal in a radio-frequency discharge plasma

By employing the particle-in-cell method we study the distributions of the electric field strength and of the electron and ion concentrations in the microparticle crystal in the electrode sheath in a radio-frequency discharge in helium. The coordinates and charges of the microparticles are found from the balance condition for the forces acting on the particles and the balance of electron and ion fluxes to the particles. With periodic boundary conditions introduced, we investigate the three-dimensional problem for the unit cell of the microparticle crystal. We examine the dependence on gas pressure and discharge voltage of the main crystal parameters: the critical particle separation (at which a phase transition from a monolayer crystal to a double-layer crystal occurs), the particle potentials, and the distances between the layers in the double-layer crystal. We obtain the critical values of the friction coefficient for the particles in the gas, i.e., values below which the crystal becomes unstable against the development of particle oscillations in the transverse direction, and compare the experimental data on crystal structure and stability with the theoretical results. Finally, we set up an approximate model that makes it possible to calculate the main parameters of the microparticle crystal. Zh. éksp. Teor. Fiz. 115, 877–893 (March 1999)     

The density of phonon states in 2D systems

The density of phonon states is calculated by the molecular dynamics method both in solid and liquid phase for three types of two-dimensional systems: Coulomb, dipolar and Lennard-Jones. The plasma-like peaks are observed in Coulomb and dipolar systems. The Lennard-Jones system has a diffusive maximum and a high-frequency tail. Melting of the systems changes the shape of the spectrum due to the appearance of diffusion.     

Interactions between cyclic carbon clusters

This work reports on the results of MINDO/3 calculations of the structures and energies of monocyclic and bicyclic carbon clusters. Interactions between the rings and between the cyclic and linear clusters are considered. For the most typical reactions (cyclization of linear clusters and insertion of chains into rings and formation of bicyclic structures), the energy barriers are estimated. The bond energies of the bicyclic structures depend on the parities of the initial rings. The most stable configurations with bond energies of the order of 5 eV result from coupling of the odd-membered rings. It is shown that typical condensation conditions correspond to the absorption of the linear clusters by the cyclic structures. Institute of Theoretical and Applied Mechanics, Siberian Branch, Russian Academy of Sciences. Institute of Computational Technologies, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 6, pp. 991–997, November–December, 1995. Translated from I. Izvekova     

Maturation of auditory temporal integration and inhibition assessed with event-related potentials (ERPs)

Background

We examined development of auditory temporal integration and inhibition by assessing electrophysiological responses to tone pairs separated by interstimulus intervals (ISIs) of 25, 50, 100, 200, 400, and 800 ms in 28 children aged 7 to 9 years, and 15 adults.

Results

In adults a distinct neural response was elicited to tones presented at ISIs of 25 ms or longer, whereas in children this was only seen in response to tones presented at ISIs above 100 ms. In adults, late N1 amplitude was larger for the second tone of the tone pair when separated by ISIs as short as 100 ms, consistent with the perceptual integration of successive stimuli within the temporal window of integration. In contrast, children showed enhanced negativity only when tone pairs were separated by ISIs of 200 ms. In children, the amplitude of the P1 component was attenuated at ISIs below 200 ms, consistent with a refractory process.

Conclusions

These results indicate that adults integrate sequential auditory information into smaller temporal segments than children. These results suggest that there are marked maturational changes from childhood to adulthood in the perceptual processes underpinning the grouping of incoming auditory sensory information, and that electrophysiological measures provide a sensitive, non-invasive method allowing further examination of these changes.     

Isomerization kinetics of carbon clusters 1. Potential of interatomic interaction

Formation of fullerenes — spherical carbon clusters C₆₀ and clusters of other sizes — during condensation of carbon vapors has not yet received theoretical explanation. Recent experimental works concerned with cluster formation in carbon vapors have established that during condensation carbon atoms form rings and then polycyclic clusters, which are precursors of fullerenes. Theoretical investigation of the spontaneous formation of fullerenes from polycyclic rings calls for a simple model of the potential of interatomic interaction of carbon, which would allow fast calculations of bond energies and statistical sums of the clusters. We use the modified Brenner potential, which was developed for hydrocarbon molecules. The parameters of the potential are refitted according to the results of quantum chemical calculations. Institute of Theoretical and Applied Mechanics, Siberian Branch, Russian Academy of Sciences. Institute of Computation Technologies, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Struktumoi Khimii, Vol. 37, No.4, pp. 664–670, July–August, 1996. Translated by I. Izvekova     

Instability and melting of a crystal of microscopic particles in a radio-frequency discharge plasma

The development of instability, heating, and melting of a two-layer crystal of dust particles in the sheath of a radio-frequency discharge has been studied by the method of Langevin molecular dynamics. The interaction forces between particles are determined in terms of a model developed earlier, in which the ion clouds under the upper particles are replaced by effective point charges. Both a pure Coulomb interaction and a screened interaction are considered. Various regimes of particle motion in the crystal are discussed. The experimental and calculated results for the mean energy of particles and the number of defects in the crystals are compared. Zh. éksp. Teor. Fiz. 114, 1672–1690 (November 1998)     

Melting in a two dimensional system with dipole interaction

Molecular-dynamics simulations show that the solid-liquid phase transition in a two-dimensional system of dipoles is of first order. The translational and orientational correlation function, the static form-factor and the dielectric function are calculated. We analyze also the self-diffusion coefficient. It is seen that self-diffusion takes place mainly on the cluster boundaries.